Thin film Bulk Acoustic Wave (BAW) resonators utilizing the thickness longitudinal resonance of a piezoelectric (PZ) film have emerged as a viable alternative to surface acoustic wave devices and quartz crystal bulk acoustic resonators for mobile communication and high-speed serial data applications. RF front-end BAW filters/duplexers offer superior filtering characteristics such as low insertion loss and sharp roll-off, power handling, and electrostatic discharge (ESD) robustness. High frequency oscillators based on ultra small factor temperature compensated BAW resonators have been shown to demonstrate excellent phase noise, wide tuning range and low power dissipation. Additionally, these miniaturized thin film resonators are fabricated in CMOS-compatible processes on silicon substrate, allowing low unit cost and promising eventual integration with CMOS.
A BAW resonator includes an acoustic mirror and two electrodes between which a PZ layer is arranged, which is called a piezoelectric excitation portion. The lower and upper electrodes also serve as feeding or excitation electrodes to cause a mechanical oscillation in the stacked layers. The acoustic mirror provides acoustical isolation between the BAW structure and a substrate.
FIG. 9 shows a top view of a conventional BAW resonator. The acoustic mirror 82 comprising a selected number of alternating high and low acoustic impedance layers is deposited on the substrate, which is used to transform the acoustic impedance of the substrate approximately into that of air. The major portion of bottom electrode 84 is disposed at an inner side of the contour of the acoustic mirror 82. At the connection edge 88, some portion of the top electrode 86 has to cross over the bottom electrode 84.
Performance of the thin film BAW resonator can be represented by the effective electromechanical coupling coefficient (Kt2) and the quality (Q) factor. The greater the effective Kt2 becomes, the wider the bandwidth of a RF filter or the tuning range of a voltage controlled resonator can be made. It is important that the resonator should be prepared by employing the PZ thin film having the high intrinsic Kt2 and aligning the polarization axis of the PZ film to the direction of the thickness of the film, in order to maximize the effective Kt2. The Q factor relates to the insertion loss when the RF filter is formed, and to the purity of the oscillation of the voltage controlled oscillator. While the oscillation relates to various energy loss mechanisms such as acoustical damping (material losses) and laterally escaping waves determined by boundary conditions of the resonator, high purity of the PZ film exhibiting good columnar grain structures with highly preferred c-axis orientation is prerequisite to achieve good performance of BAW devices. It is known that the texture of the PZ film is strongly dependent on both the roughness and the texture of the underlying electrode upon which it is deposited. A smooth underlayer with a sharp texture is the best possible combination. When the PZ layer is deposited, it follows the terrain of the underlayer and has a tendency to crack when layered over sharp topography, for example, on electrode layer that has a nearly vertical edge making an abrupt end. Cracks in the PZ layer significantly decrease the ESD robustness of resonator.
FIG. 10 shows a top view of a conventional BAW resonator as disclosed by U.S. Pat. No. 6,384,697 to Ruby et al. In the BAW resonator, a method to support an acoustic resonant portion on a substrate is provided. The acoustic resonant portion comprising a PZ layer sandwiched between a bottom electrode 94 and a top electrode 96. In practice, at least one side of the top electrode 96 has to extend beyond the contour of the acoustic mirror to connect with pads or other circuits. The bottom electrode 94 spans the entirety of the cavity 92 functioning as an acoustic mirror. This approach avoids cracks of PZ layer in the free standing membrane and improves the mechanical reliability of resonator. However, the voids or cracks in PZ layer when deposited over the edge of its underlying electrode 94 lead to serious susceptibility to electrostatic discharges. The effective Kt2 of the resonator is reduced when some portion of electrodes sandwiching PZ layer are in contact with substrate.
A tapered end portion of the bottom electrode could be formed in order to prevent cracking and discontinuity in the PZ layer. FIG. 11 shows a cross sectional view of a conventional bulk acoustic wave resonator having a tapered end portion of the bottom electrode. The BAW resonator comprises an acoustic mirror 1120 formed on the top surface of the substrate or in the substrate 1110, and two electrodes 1140 and 1160 between which a piezoelectric layer 1150 is sandwiched. The tapered end portion 1142 of the bottom electrode 1140 could be within or outside (or partially outside) of the contour of the acoustic mirror 1120. The tapered end portion 1142 of the bottom electrode 1140 is typically formed with dry plasma or wet chemical etching process. Compared to other regions not exposed to etching, the etching damaged electrode area has worse grain structure and the etched surface in the tapered end portion area 1142 is much rougher. Both of the Q factor and the effective Kt2 of the resonator formed with the PZ layer deposited in a region 1166 overlapping the tapered end portion 1142 with high surface roughness deteriorate remarkably.
In addition, as disclosed in U.S. Pat. No. 6,924,717 to Ginsburg et al., forming a tapered bottom electrode requires good control of a slop angle and increases the process complexity and manufacturing cost (e.g., the dry etching being excessively long).
People skilled in the art widely recognize that acoustic energy in the form of lateral modes can leak out from sides of the resonator and escapes into the supporting substrate. The acoustic boundary condition at the periphery of the resonator has to be optimized to avoid generating those energy consuming lateral modes. In particular, the acoustic energy escaping from the connection edge region 88 and 1166, as shown in FIGS. 9 and 11, respectively, associated with crossing the bottom electrode with top electrode is identified as one of the dominating sources of loss. It is important to minimize the interaction of lateral modes with the connection edge 88 of the resonator.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.